Page 489 - Physics Coursebook 2015 (A level)

Chapter 30: Quantum physics
a Energy 0
We can calculate the frequency f and wavelength λ of the emitted electromagnetic radiation.
The frequency is:
f = E = 1.64 × 10−18
h 6.63 × 10−34 f = 2.47 × 1015 Hz
The wavelength is: λ= c = 3.00×108
f 2.47 × 1015
λ = 1.21×10−7 m = 121nm
This is a wavelength in the ultraviolet region of the electromagnetic spectrum.
QUESTION
b Energy 0
photon absorbed
Figure 30.16 a When an electron drops to a lower energy level, it emits a single photon. b A photon must have just the right energy if it is to be absorbed by an electron.
Photon energies
When an electron changes its energy from one level E1 to another E2, it either emits or absorbs a single photon. The energy of the photon hf is simply equal to the difference in energies between the two levels:
photon energy = ΔE hf = E1 − E2
or hc=E1−E2 λ
Referring back to the energy level diagram for hydrogen (Figure 30.15), you can see that, if an electron falls from the second level to the lowest energy level (known as the ground state), it will emit a photon of energy:
photon energy = ΔE
hf = [(−0.54) − (−2.18)] × 10−18 J hf = 1.64 × 10−18 J
14
Figure 30.17 shows part of the energy level diagram of an imaginary atom. The arrows represent three transitions between the energy levels. For each of these transitions:
a calculate the energy of the photon
b calculate the frequency and wavelength of
the electromagnetic radiation (emitted or absorbed)
c state whether the transition contributes to an emission or an absorption spectrum.
photon emitted
not absorbed
Energy / 10–18 J 0
–0.4
–1.7 –2.2
–3.9
–7.8
b
c
a
Figure 30.17 An atomic energy level diagram, showing three electron transitions between levels – see Question 14.
Not to scale
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